Today I received my review copies of new D&D 4e releases "Martial Power 2" and "Underdark". I will be reading these and writing reviews shortly. In particular, "Underdark" looks like an interesting take on one of D&D's staple adventure environments. It looks like there would be a lot to use in the 4e version of the underdark even for groups using an older iteration of the rules.
Martial Power 2, on the other hand, is probably exhibit A in the hands of those who argue that D&D 4e has far too many books already. As I have stated before (in this post and its comments), it only costs $10 for a player to download the character builder and have all the information pertaining to character advancement ever published in handbooks, power books, Dragon magazine, adventures... That really isn't an exorbitant buy in, certainly not compared to the $20 - $30 most games require (at the cheap end) of a player who wants her own set of the rules she will be using.
I see the D&D 4e hardbacks that Wizards is producing as being aimed at the collector types who want to own every book. Old school collector types amass collections of expensive D&D books at least as much as 4e collector types. The casual player will really not ever need more than the $10 D&D Insider subscription fee to play and have a wealth of data at her fingertips.